1. Field of the Invention
The present invention relates to the field of optical recording and playback and, more particularly, it relates to improvements in apparatus for both focusing a laser beam on the surface of a data storage element, such as an optical disk or tape, and controlling the radial position of the focused beam on the element so that information can be written (or read from) a desired data track.
2. Discussion of Prior Art
Heretofore, a wide variety of focus/tracking actuators have been proposed and utilized in the write/read heads of optical and magnetooptical disk drives to both maintain proper focus of a laser beam on a rapidly spinning optical recording element and control the position of the focused spot with respect to a desired data track. Typically, such actuators comprise a pair of "moving coil" linear motors which serve to adjust the position of a movably mounted objective lens along two orthogonal axes, for example, along a vertical axis to adjust the beam focus, and along a horizontal axis to control the tracking position of the beam. Each of these linear motors comprises a tiny coil of wire which is secured to the lens housing and adapted to move therewith, and one or more permanent magnets rigidly mounted on the actuator housing for producing a magnetic field which interacts with the electromagnetic field created in the vicinity of the coils when a current in the coils is caused to flow. The respective directions of the interacting fields effects movement of the lens housing along the desired orthogonal axes.
While moving coil linear motors have proven highly effective in focus/tracking actuators for controlling beam focus and placement, they are not without disadvantages. For example, due to location of the focus and tracking coils on the moving payload, flexible cables or wires must be used to deliver current to the coils. As a result, manufacturing processes become more complicated, and the useful life of these wires becomes an issue. Another disadvantage of moving coil actuators is related to the fact that some portions of focus and tracking coils lack support that quite often results in dynamic disturbances especially at high frequencies. These disadvantages are not present in focus/tracking actuators employing linear motors of the "moving-magnet" type. In such motors, the focus and tracking coils are stationary and the permanent magnet(s) moves with the motor payload, i.e., the lens housing. However, focus/tracking actuators with moving magnets have not gained wide-spread acceptance because they tend to be less efficient, in terms of power consumption, in accelerating the payload. In most designs, the lack an adequate flux carrying steel structure results in a weak magnetic field. Also, size limitation of these devices is required to diminish the attraction of the moving magnets to the surrounding steel. In extreme cases, some motors do not incorporate steel parts at all, and magnetic flux is passing through air.
Among various focus/tracking actuators with moving-magnet motors, one design is described in U.S. Patent No. 5,136,558. In this design, a pair of permanent magnets are attached to opposite sides of a movably mounted lens holder, and two pair of focus and tracking coils are arranged on a stationary base so that the respective electromagnetic fields created by the energization of the coils interact with the magnetic flux of the permanent magnets and operate to move the lens in focus and tracking directions. Here, the focus coils are circular in configuration, each having a central axis (about which the coil wire is wound) which is concentric with the lens axis. The tracking coils are elongated in shape, each having a central axis which is perpendicular to the lens axis. In this design, no provision is made, e.g., in the form of a steel structure, for facilitating the return of magnetic flux passing from one pole of the magnet, through the coil, and back to the opposite pole of the magnet. Such return is effected through the surrounding air, a medium of relatively high magnetic reluctance. As a result, this particular design is somewhat inefficient in converting electrical power to payload acceleration.
Another moving-magnet type focus/tracking actuator is disclosed in U.S. Pat. No. 4,759,005. Here, a more magnetically efficient lay-out of a moving magnet motor is utilized but at the expense of doubling the amount of focus and tracking coils and winding each of them in a planar fashion.